This invention relates to burner control systems and, more particularly, to fail safe burner control systems that provide a purge period prior to ignition.
Extensive efforts have been directed toward the improvement of control systems for fuel burners such as gas and oil burners and the like. Increased system safety and reliability have been primary objectives of such efforts. These objectives, however, generally conflict with an obvious desire to limit the cost and physical size of the systems. Thus system complexity is an important consideration.
Such systems often ignite the fuel with a spark igniter. Interest has recently been directed toward systems that extinguish the spark after ignition to eliminate ratio frequency interference. However, circuits to extinguish the spark have greatly added to the complexity of the control circuit. This is particularly true since it is required that if flame is lost for any reason, the system must respond in one of two ways. Either the valve must be closed to stop the flow of fuel, or, as is preferable if heat is still required, the ignition apparatus must be reactivated in an effort to re-establish flame.
In addition, most burner systems must employ fuel supply valves that are controlled by flame sensing mechanisms which automatically interrupt fuel flow in response to a predetermined loss of flame condition. In accordance with the above requirements, circuits have been designed wherein the spark apparatus is responsive to the flame sensor so that when flame is detected, the igniter is stopped and upon loss of flame the igniter is activated to re-establish flame. A difficulty encountered with these circuits is their complexity; for example, often a plurality of feedback loops, or the like, are used. A danger in having such a complex system is that failure of one or more circuit components can cause an unsafe condition as, for example, a situation in which the valve remains open but the ignition apparatus is not activated. An explosive amount of fuel may thereby enter the combustion chamber.
Many conventional circuits provide a capacitor flame sensor that is charged by flame rectified current and a valve that opens when the charge on the capacitor exceeds a predetermined minimum. To initiate operation of some circuits of this type, the capacitor is precharged to open the valve and is kept charged by the rectified current if flame is achieved. If no flame is achieved before the capacitor becomes discharged, the valve closes and the system shuts down. The unsafe condition can occur in this circuit, for example, if flame is lost or never established, but a malfunction in the precharging circuit keeps the capacitor charged and thus the valve open. Other problems in circuits of this type result from line powered amplification stages utilized before the low energy flame signal and the valve control circuit responsive thereto. In certain instances, the line power can introduce false signals not distinguishable from the flame signal and therefore effective to cause improper and sometimes dangerous operation.
Many of the above problems are eliminated in a burner control system disclosed in U.S. Pat. No. 3,853,455. That system automatically activates an igniter to re-establish flame in the event of a loss thereof, but closes a fuel supply valve if a failure to establish flame occurs, either initially or after loss of flame. In addition, the system described is fail-safe; that is not subject to unsafe operation as a result of malfunction of either any component or any group of components.
Despite these advantages, certain characteristics of the system described in the above noted patent could be improved. For example, desirable improvements would be a capability for more rapid switching between purge and ignition periods, lower cost, greater immunity to circuit noise, and improved operational stability.
The object of this invention, therefore, is to provide an improved fuel burner control system.